1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and apparatus for cell selection in a mobile communication system supporting a Hierarchical Cell Structure (HCS).
2. Description of the Related Art
In general, a mobile communication system includes a plurality of cells, the cells corresponding to small-sized service areas divided according to Node Bs from the entire service area of the mobile communication system. Each of the Node Bs is controlled in a concentrated manner by a Radio Network Controller (RNC), so that a subscriber can continuously perform the communication while moving between cells. In a mobile communication system, when a User Equipment (UE) escapes from an area of one Node B and enters an area of another Node B, the UE can continuously perform the communication by the handoff function.
A mobile communication system supporting hierarchical cells discriminates between cells depending upon the serviceable area in providing the communication service. In the mobile communication system supporting hierarchical cells, the cells include various types of cells, such as a macro cell, a micro cell, and a pico cell, in order to effectively provide discriminated services according to the areas. The macro cell provides the widest service area, supports fast movement, and has a low data rate. The micro cell provides a narrow service area, supports slow movement, and has a high data rate.
The pico cell is usually used in order to provide a communication service within a building in order to provide a communication service to a particular area, such as a campus, a playground, an airport, and a shopping mall, in order to provide a communication service when a special event or a natural disaster has occurred, in order to provide a communication service to an area in which remote control is impossible, or in order to assist the communication service of a macro cell or a mini cell having a tunnel, or in order to improve the communication quality of an area having a bad communication quality.
Meanwhile, the UE measures reception levels of multiple pilot channels, and performs cell selection based on the measured values. That is, in a cell network having overlapping cells, the UE performs cell selection based on the reception levels. Usually, the UE performs the cell selection by a Received Signal Strength Indicator (RSSI). In a boundary region between two cells, the UE detects an RSSI of a serving cell to which the UE currently belongs, and RSSIs of multiple neighbor cells adjacent to the serving cell. When an RSSI of a neighbor cell is larger than the RSSI of the serving cell, the UE selects the neighbor cell.
Further, when system information or an inner trigger of the UE changes, cell reselection is estimated. The “quality measurement” performed for the cell reselection is measured by a Node B. Further, the “quality measurement” is performed so as to satisfy requirements of the Radio Access Technology (RAT) quality, inter-frequency, or intra-frequency according to the threshold within the system information and the quality level of the serving cell. In a mobile communication system supporting the hierarchical cell structure, a cell with a higher priority (HSC_PRIO) is considered because it is possible to reduce the number of measured cells. At this time, the “quality measurement” includes measurement of the speed of the UE, which will be described hereinafter.
First, determination that the UE moves at a high speed is expressed by formula (1) below.High-Mobility state: # of re-selection>>NCR during TCRmax  (1)
In formula (1), NCR refers to a predetermined maximum number of times for cell reselection, TCRmax refers to a time period for estimating cell reselection, and # of re-selection refers to the number of times of cell reselection. When it is concluded from formula (1) that the number of times of cell reselection is larger than the predetermined maximum number of times for cell reselection during the time period for estimating cell reselection, the Node B determines that the speed of the UE is high. Based on formula (1), the Node B performs the “quality measurement” for only cells having the same priority as or a lower priority than that of the serving cell.
Next, determination that the UE escapes from the high speed state is expressed by formula (2) below.leave the High-Mobility state: # of re-selection≦NCR during TCRmax after TCRmaxhyst  (2)
In formula (2), NCR refers to a predetermined maximum number of times for cell reselection, TCRmax refers to a time period for estimating cell reselection, and TCRmaxhyst refers to an additional time period before the UE returns to the low speed state. Here, TCRmaxhyst is a factor added to TCRmax and implies that application of more time is necessary in order to determine that the UE is in a low speed state. When it is concluded from formula (2) that the number of times of cell reselection is smaller than or equal to the predetermined maximum number of times for cell reselection during the time period for estimating the cell reselection after the additional time period before the UE returns to the low speed state, the Node B determines that the UE has escaped from the high speed state.
The “High-Mobility state” in formula (1) can be applied to layers showing a clear difference in signal intensity due to the high priority cell (HCS_PRIO). However, an error may occur when it is applied to cells of the same layer.
Further, it is problematic to use the number of times of cell reselection in determining if the UE is in the high speed state or low speed state. In a boundary region in which cells overlap or an area having a bad signal intensity in which the signaling null occurs, the UE may switch between two or three Node Bs within a short time interval due to the ping-pong effect. This phenomenon is more severe in a micro cell having a narrow cell overlapping area, and may cause a user to erroneously select a macro cell as a serving Node B even when the user is not a high speed user.
Currently, a filtering technique is applied in order to prevent the ping-pong phenomenon in a fast fading situation. If the UE is moving at a high speed (for example, more than 50 km/h), it is relatively easy to average the fast fading. That is, the filtering time is about 100 ms, which is sufficient. However, if the UE is moving at a low speed, that is, when the UE is in a pedestrian environment, a filter having a size of one second is necessary in order to average the fast fading. In this way, by extending the filtering time, the UE can proceed to the area of the neighbor cell before the cell reselection occurs. This may cause high interference to other users, which may decrease the system capacity.